Selective permeable membrane for separation of liquid solution

ABSTRACT

A novel selective permeable membrane is made by cross-linking of chitosan by aldehyde, preferably by dialdehyde. The dialdehyde cross-linked membrane has a remarkably high separation efficiency for water-alcohol solution due to hydrophilic property of chitosan and its very dense structure obtained by the cross-linking of chitosan with dialdehyde.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a selective permeable membrane forseparation of a liquid solution, and more particularly to such amembrane adapted in use to selectively separate a particular volatilecomponent from its liquid solution containing another volatilecomponent, for example, to separate the water from a water-alcoholmixture solution.

2. Description of the Prior Art

In order to separate a liquid solution such as a water-alcohol solution,it has been proposed to use a selective permeable membrane particularlysuitable to a pervaporation method in which the membrane is in contactwith the liquid solution for separation of the liquid solution. Includedin the membrane for separation of water-methanol mixture is a cellophanemembrane as proposed in the article "Japanese Polymer Science Symposium,No. 41, 145" in 1973, and a graft polyvinyl alcohol membrane as proposedin the article "Japanese Polymer Science Vol. 26, 3223" in 1981.Further, Japanese Patent Publication (KOKAI) No. 62-4407 discloses achitosan-vinyl monomer membrane for separation of water-ethanol mixture,and Japanese Patent Publication (KOKAI) No. 62-7403 discloses a chitosansalt membrane for separation of water-alcohol mixture.

Unfortunately, these membranes have been found to be insufficient in itsseparation efficiency and failed to come into industry practice. Theseparation efficiency of the membrane can be evaluated in terms of aseparation factor α, which indicates that the membrane allows aparticular component of the liquid solution to selectively permeatetherethrough when α being greater than 1 and indicates a greaterseparation efficiency with increasing value of α, as discussedhereinafter. The above membranes without chitosan show a separationfactor only in the order of several tens and even the membranesincluding chitosan cannot have the separation factor increased to asufficient level for commercial practice.

SUMMARY OF THE INVENTION

The present invention eliminates the above insufficiency in theseparation of liquid solution and provides a unique permeable membranemade of a chitosan derivative having superior separation efficiency.Chitosan is a cationic polymer obtained by deacetylation of chitin andsoluble in a dilute acid. Chitin is a straight chain polysaccharideforming the supporting structure of crustacea, fungi, and the likeavailable from abundant natural resources. In accordance with thepresent invention, chitosan is cross-linked by aldehyde to form aselective permeable membrane for liquid separation which has anincreased separation efficiency not expected by the prior membraneprepared by simple deacetylation of chitosan. Such increased separationefficiency is assumed to come from an increased density of the membraneachieved by the cross-linking. In addition, due to the hydrophilicproperty of chitosan, the dialdehyde cross-linked chitosan membraneshows superior separation efficiency particularly for water-and-organicsolvent solution. Included in aldehyde is a dialdehyde which isprimarily utilized in the present invention for cross-linking ofchitosan by dehydration-condensation of reactive amino groups containedin the molecule of chitosan and aldehyde groups of dialdehyde, as shownin the following formula: ##STR1## wherein n represents an integer of200 to 20,000, and R represents a group of formula:

--(CH₂)m--wherein m represents an integer of 1 or more, or ##STR2##

Monoaldehyde may be also utilized as an aldehyde for cross-linking ofchitosan wherein monoaldehyde reacts with amino groups of chitosan toform methylol groups which are to be condensed with each other bydehydration and/or condensed with other amino groups of chitosan bydehydration to effect cross-linking of chitosan.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a vessel in which a selectivepermeable membrane of the present invention is adapted in use to effectliquid separation in liquid-to-vapor phase by pervaporation method; and

FIG. 2 is a schematic view showing a vessel in which the selectivepermeable membrane of the present invention is adapted in use to effectliquid separation in vapor-to-vapor phase by evapomeation method.

DETAILED DESCRIPTION OF THE INVENTION

An aldehyde cross-linked chitosan membrane in accordance with thepresent invention can be prepared in the following methods. In onemethod, chitosan is dissolved in an aqueous solution of acetic acidwhich solution is then spread evenly on a suitable supporting surfaceand dried to form a membrane. The resulting membrane is then soaked inan aqueous solution of sodium hydroxide to be neutralized followed bybeing washed and dried to obtain a chitosan membrane. Further, thechitosan membrane is soaked in an aqueous solution of dialdehydeincluding a dilute acid as a catalyst to proceed cross-linking and isthen washed and dried to obtain the dialdehyde cross-linked chitosanmembrane having selective permeability. The other method comprises thesteps of dissolving chitosan into an aqueous solution of acetic acid,adding to this solution an aqueous solution of dialdehyde including adilute acid as a catalyst, spreading the mixture solution on a suitablesupporting surface, drying it to obtain the membrane, soaking theresulting membrane in an aqueous solution of sodium hydroxide toneutralize it, washing and drying the same to obtain the dialdehydecross-linked chitosan membrane. When monoaldehyde is utilized to obtainan aldehyde cross-linked chitosan, the same steps may be utilized toreact chitosan with monoaldehyde with the condensation of methylolgroups.

Preferably, chitosan employed in the present invention has a molecularweight of 50,000 to 5,000,000 and is deacetylated up to 60-100%.Dialdehyde utilized for the above cross-linking includes glyoxal,malondialdehyde, succindialdehyde, glutardialdehyde, adipicdialdehyde,maleindialdehyde, phthaldialdehyde, iso-phthaldialdehyde,telephthaldialdehyde, and dialdehyde starch. Included in monoaldehydeare formaldehyde, acetaldehyde, propionaldehyde. The aldehydecross-linked membrane of the present invention is preferred to have athickness not more than 200 μ.

The aldehyde cross-linked membrane thus obtained is adapted in use forliquid separation by "pervaporation (permeation-vaporation) method".FIG. 1 illustrates the principle of the "pervaporation method" whichutilizes a vessel 2 divided by the membrane 1 into an upper solutionchamber 3 and an lower vacuum chamber 4. The upper solution chamber 3 isfilled with a liquid solution 5 containing more than one volatile orevaporizable component such as water and alcohol in an alcohol-watersolution. By evacuating the lower vacuum chamber 4, a particularcomponent of the liquid solution 5 is allowed to preferentially permeatethrough the membrane 1 into the vacuum chamber 4 and is evaporatedtherefrom. Thus, the particular component can be collected as beingseparated from the liquid solution 5. In the opposite sense, the liquidsolution from which the substantial amount of the particular componentis removed can be recovered from the solution chamber 3. With thispervaporation method utilizing the permeable membrane 1, liquidseparation can be effected successfully without causing any membranepressurization as seen in the conventional reverse osmosis method.Further, the aldehyde cross-linked chitosan membrane exhibitspreferential permeability to water due to its hydrophilic property,therefore it is found most effective for separation of water-and-organicsolvent solution such as water-alcohol solution.

As an alternative liquid separation method with the use of the membrane,the inventor has proposed in the pending patent application Ser. No.130,504 filed on Dec. 9, 1987 [US], an "evapomeation (evaporationpermeation) method" which is characterized to separate a particularcomponent from its liquid solution through the membrane invapor-to-vapor phase. The principle of the "evapomeation method" isillustrated in FIG. 2 which utilizes a vessel 6 divided by the permeablemembrane 1 into an upper vacuum chamber 7 and a lower solution chamber8. A clearance or space is provided between the undersurface of themembrane 1 and the surface of the liquid solution 5 stored in thesolution chamber 8 so as to keep the permeable membrane 1 out of directcontact with the liquid solution 5. As evacuating the vacuum chamber 7,the solution chamber 8, which is in air communication therewith throughthe permeable membrane 1, is correspondingly evacuated to cause theliquid solution to evaporate. When the vapor from the liquid solutionthus evaporated comes into contact with the permeation membrane 1, it isallowed to permeate through the membrane 1 as it diffuses therein and isforced to enter the vacuum chamber 7 under the effect of the vacuum.Since the permeable membrane 1 allow a particular component toselectively permeate in preference to the others, it is readily possibleto complete the separation either by collecting the component havingpassed into the vacuum chamber 7 through the permeable membrane 1 or bycollecting the remaining solution in the solution chamber 8 from whichthe component has been removed. As seen in the above, the separation ofthe liquid solution 5 can be carried out in vapor-to-vapor phasepermeation through the membrane 1. Consequently, the membrane 1 can bekept free from substantial swelling which is inherent where the membraneis in direct contact with the liquid solution as in the abovepervaporation method, effectively preventing the permeable membrane fromlowering its permeation efficiency. It is noted in this connection thatthe membrane of the present invention shows a remarkably superiorseparation efficiency for water-alcohol separation when utilized in theabove "evapomeation separation method", or shows a maximum separationfactor α of infinite value, as discussed hereinafter with reference toexamples of the present invention.

When the permeable membrane of a reduced thickness is utilized, it ispreferred to place the membrane on a support member for reinforcingreinforce the membrane enough to resist against a negative pressuredeveloped therebehind. The support member may be in the form of a net ora porous substrate, for example, a porous unwoven fabric ofpolypropylene fibers or polyester fibers, a porous film oftetrafluoroethylene or polysulfone, or porous ceramic plate.

The present invention will be discussed with reference to the followingexamples, which are provided by way of illustration and not by way oflimitation.

EXAMPLE 1

2.0 g of chitosan powder having an average molecular weight of 50,000 to100,000 and being deacetylated up to 99.1% was dissolved into 200 g ofan aqueous solution of 1N acetic acid and stirred overnight at 25° C.The resulting solution was filtered through a glass-filter and deaired.Subsequently, 20 of the solution was spread on a silicon finished glassplate and dried at 60° C. for 5 hours to form a membrane ofchitosan-acetate. After being peeled off, the chitosan-acetate membranewas soaked in an aqueous solution of 1N sodium hydroxide overnight to beneutralized and was thoroughly washed and dried under a reduced pressureat a room temperature to obtain a 20μ thick chitosan membrane.

The chitosan membrane was soaked into a 0.4% aqueous solution ofglutaraldehyde to which an aqueous solution of 0.5 N sulfuric acid wasadded as a catalyst and left for 15 minutes at a room temperature inorder to cross-link chitosan with glutaraldehyde, which membrane wassubsequently washed and dried under a reduced pressure at roomtemperature to give a 20μ thick dialdehyde cross-linked chitosanmembrane.

The dialdehyde cross-linked membrane thus obtained was utilized as thepermeable membrane dividing the vessel 2 of FIG. 1 into the uppersolution chamber 3 and the lower vacuum chamber 4. Samples 1 to 8, whichare aqueous solutions of different ethanol concentrations, were eachintroduced into the solution chamber 3 in order to preferentiallypermeate the water through the membrane by evacuating the vacuum chamber4 down to a pressure of 10⁻² Torr. at a solution temperature of 40° C. Aseparation factor α and a permeation rate Q were obtained for eachsample for evaluation of permeation efficiency of the membranae, aslisted in Table 1.

The separation factor α in Table-1 is introduced to indicate the weightratio of a particular component being separated to the liquid solutionfrom which the particular component is separated and is accordinglydefined in this instance by the following equation: ##EQU1## whereinX_(H20) and X_(ETOH) are fractions of water and ethanol respectively inaqueous ethanol solution received in the solution chamber, while Y_(H20)and Y_(ETOH) are fractions of water and ethanol, respectively afterpermeating through the membrane as measured in the subsequentlyliquified phase. As apparent from the above relation, when α is greaterthan 1, it means that the water has passed through the membrane in agreater amount than the ethanol and that the water is allowed to passpreferentially through the membrane to a larger extent as the value αbecomes greater. It is noted in this connection that the permeation rateQ in Table 1 is defined as a rate at which the water and ethanol havingthe proportion represented by the value has permeated through themembrane per unit time and unit area.

                                      TABLE 1                                     __________________________________________________________________________    ethanol                                                                       concentration                                                                            Example 1       Comparative Example 1                                  [wt %] in                                                                            separation                                                                          permeation                                                                              separation                                                                          permeation                                       water-ethanol                                                                        factor                                                                              rate      factor                                                                              rate                                         sample                                                                            solution                                                                             [α]                                                                           [kg/m.sup.2 hr]                                                                     α × Q                                                                 [α]                                                                           [kg/m.sup.2 hr]                                                                     α × Q                      __________________________________________________________________________    1   0      1     1.37  1.37                                                                              1     1.87  1.87                                   2   10     2     1.27  2.54                                                                              0.6   1.50  0.90                                   3   30     10    0.78  7.80                                                                              2     1.36  2.83                                   4   50     52    0.90  46.8                                                                              13    0.67  8.38                                   5   70     182   0.39  71.0                                                                              50    0.35  17.5                                   6   90     505   0.06  30.3                                                                              31    0.12  3.75                                   7   95.6   390   0.03  11.7                                                                              17    0.06  1.03                                   8   100    1     0.02  0.02                                                                              1     0.03  0.03                                   __________________________________________________________________________

Comparative Example 1

The chitosan membrane before being cross-linked with glutaraldehyde inExample 1 was utilized as the membrane to carry out the separation ofwater-alcohol solution in the same manner as in Example 1. Like samplesof different ethanol concentrations were each tested to provide theseparation factor α and permeation rate Q under the same conditions asin Example 1. The test results are also listed in Table 1.

As apparent from Table 1, it is confirmed that Example 1 utilizing thedialdehyde cross-linked chitosan membrane shows the separation factor ofup to several hundreds, while Comparative Example 1 utilizing the simplechitosan membrane shows the separation factor in the order of severaltens, which demonstrates that Example 1 has superior separationefficiency of preferentially separating the water from the alcohol-watersolution as compared to Comparative Example 1.

EXAMPLE 2

The same dialdehyde cross-linked chitosan membrane as obtained inExample 1 was utilized as a permeable membrane to effect liquidseparation for water-ethanol solution in accordance with the"evapomeation method" utilizing the vessel 6 of FIG. 2. Like samples 1to 8, which are aqueous solutions of different ethanol concentrations,were each introduced into the solution chamber 8 in order topreferentially permeate the water through the membrane by evacuating thevacuum chamber 7 down to a pressure of 10⁻² Torr. at a solutiontemperature of 40° C. A separation factor β and a permeation rate Q wereobtained for each sample for evaluation of permeation efficiency, aslisted in Table 2.

Comparative Example 2

The same chitosan membrane as obtained in Example 1 before beingcross-linked with glutaraldehyde was utilized to effect liquidseparation in the like manner as in Example 2 to present the separationfactor α and the permeation rate Q for each sample, as listed in Table2. In Table 2, the values enclosed by braces for the column "ethanolconcentration [wt %]in water-ethanol solution" represent respectiveconversion values for ethanol concentration [wt %]in vapor, and the likeconversion values are shown as enclosed by braces for each column of"separation factor " and "α×Q".

                                      TABLE 2                                     __________________________________________________________________________    ethanol                                                                       concentration                                                                            Example 2        Comparative Example 2                                 [wt %] in                                                                            separation                                                                          permeation separation                                                                          permeation                                      water-ethanol                                                                        factor                                                                              rate       factor                                                                              rate                                        sample                                                                            solution                                                                             [α]                                                                           [kg/m.sup.2 hr]                                                                     α × Q                                                                  [α]                                                                           [kg/m.sup.2 hr]                                                                     α × Q                     __________________________________________________________________________    1   0        1   0.24  0.24  1    0.18  0.18                                  2   10      219  0.20  43.8  5    0.15  0.75                                      (43.9) (1542)      (308.4)                                                                             (33)       (4.95)                                3   30     ∞                                                                             0.15  ∞                                                                             7    0.13  0.91                                      (60.4) (∞)   (∞)                                                                           (25)       (3.25)                                4   50     ∞                                                                             0.11  ∞                                                                             26   0.10  2.60                                      (67.7) (∞)   (∞)                                                                           (56)       (5.6)                                 5   70     ∞                                                                             0.06  ∞                                                                             37   0.04  1.48                                      (77.3) (∞)   (∞)                                                                           (53)       (2.12)                                6   90     2557  0.03  76.7 114   0.02  2.28                                      (90.8) (2797)      (83.9)                                                                             (124)       (2.48)                                7   95.6   2208  0.04  83.3 202   0.01  2.02                                  8   100      1   0.03  0.03  1    0.01  0.01                                  __________________________________________________________________________

As seen from Table 2, Example 2 utilizing the dialdehyde cross-linkedmembrane of the present invention shows greater values both in theseparation factor α and permeation rate Q with respect to every samples1 to 8 of differing ethanol concentrations than Comparative Example 2utilizing simple chitosan membrane. Particularly, with regard to samples3 to 5 for water-ethanol solutions having ethanol concentrations of 30to 70 wt%, the separation factor α is found to be ∞ indicating that 100%of the water is allowed to permeate through the membrane in preferenceto ethanol. Accordingly, the dialdehyde cross-linked chitosan membraneis confirmed to have a remarkably high separation efficiencyspecifically for ethanol-water solution having such ethanolconcentrations. From the comparison between the results of Table 1 andTable 2, it is also concluded that the dialdehyde cross-linked membraneexhibits a superior separation efficiency when utilized in the"evapomeation method" than in the "pervaporation method".

What is claimed is:
 1. A process of selectively separating water from aliquid solution containing water and an organic solvent, which comprisescontacting the solution with a permeable membrane made of chitosancross-linked with a dialdehyde and having the following general formula:##STR3## wherein n represents an integer of 200 to 20,000, and Rrepresents a group of formula:--(CH₂)_(m) --wherein m represents aninteger of 1 or more, or ##STR4## whereby water selectively vaporizesand permeates the membrane.
 2. A process according to claim 1, whereinthe chitosan has a molecular weight of between 50,000 to 5,000,000 andis 60 to 100% deacetylated.
 3. A process according to claim 1, whereinthe dialdehyde is a glyoxal, malondialdehyde, succindialdehyde,glutardialdehyde, adipicdialdehyde, maleindialdehyde, phthaldialdehyde,isophthaldialdehyde, telephthaldialdehyde or dialdehyde starch.
 4. Aprocess of selectively separating water from an aqueous solutioncontaining ethyl alcohol, which comprises contacting the solution with amembrane made of chitosan cross-linked with a dialdehyde and having thefollowing general formula: ##STR5## wherein n represents an integer of200 to 20,000, and R represents a group of formula:--(CH₂)_(m) --whereinm represents an integer of 1 or more, or whereby water selectivelyvaporizes and permeates the membrane.
 5. A process according to claim 4,wherein the chitosan has a molecular weight of between 50,000 to5,000,000 and is 60 to 100% deacetylated.
 6. A process according toclaim 4, wherein the dialdehyde is a glyoxal, malondialdehyde,succindialdehyde, glutardialdehyde, adipicdialdehyde, maleindialdehyde,phthaldialdehyde, isophthaldialdehyde, telephthaldialdehyde ordialdehyde starch.